Conventional Hall-Heroult cells for the electrolytic production of aluminum employ a carbon cell bottom which serves to supply current to a deep pool of molten aluminum forming the cathode. The cathodic aluminum is necessarily thick (at least 80-100 mm) because carbon is non-wettable by molten aluminum and during operation would not completely cover the carbon if the aluminum layer were thinner. In the conventional arrangement, a horizontal steel conductor bar is embedded in the lower part of the carbon cell bottom for the supply of current from an external source. Thus, the entire cell bottom in contact with the molten aluminum cathode consists of carbon which, in operation, is impregnated with sodium species and other ingredients of the cryolite leading to the formation of toxic compounds including cyanides. Despite the many disadvantages associated with carbon as cathode current feeder material (not-wettability by aluminum, necessitating deep pool operation; the relatively high electrical resistance of carbon, leading to energy losses; reactions within the cell environment necessitating disposal of large quantities of contaminated carbon when the cell bottom is renewed; swelling, which must be compensated by supporting the cell sidewalls in cradles, etc.) attempts to replace carbon with theoretically more advantageous materials and employing new cell designs have not so far met with success.
Thus, for example, the aluminum production cell having an electrically non-conductive refractory lining with a "bottom entry" current collector is described in U.S. Pat. No. 3,287,247. The inner end of the current collector has a cap of TiB.sub.2 projecting into a depression containing a deep pool of molten aluminum. U.S. Pat. No. 3,321,392 describes a similar arrangement in which the protruding ends of TiB.sub.2 conductor bars are rounded. U.S. Pat. Nos. 3,093,570 and 3,457,158 disclose similar designs in which bottom-entry cylindrical current collector bars or posts of TiB.sub.2 or graphite extend through a non-conductive refractory lining consisting throughout of powders of alumina and cryolite or aluminum fluoride.
U.S. Pat. No. 4,613,418 has proposed an aluminum production cell with an alumina potlining in which bottom-entry current collectors are embedded and extend to a recess in the potlining. To prevent the unwanted collection of sludge in these depressions, this patent purposes filling the depressions with balls of aluminum-wettable material. Related designs are proposed in U.S. Pat. No. 4,612,103.
These alternative cell designs, using a non-carbon cell bottom, have great promise. Replacement of the carbon cell bottom with, e.g., alumina leads to potential savings in materials and operating costs. However, such proposals heretofore have generally relied on the use of a family of materials known as Refractory Hard Metals ("RHM") encompassing the borides and carbides of metals of Group IVB (Ti, Zr, Hf) and VB (V, Nb, Ta) of the periodic table of the elements. TiB.sub.2 has been identified as the most promising RHM material. The use of these materials as part of the current supply arrangement has encountered a number of problems including cost and the difficulty of producing and machining large pieces of the materials. Such difficulties have led to the design expedients proposed in the aforementioned U.S. Pat. Nos. 4,613,418 and 4,612,103, where, for example, small pieces of TiB.sub.2 are assembled or packed together in an environment of molten aluminum as part of the current supply arrangement.
The problems experienced with RHM current collectors and further expedients for dealing with them, namely the provision of a protective barrier incorporating a molten fluoride- or chloride-containing salt mixture or a getter such as particulate aluminum, are further described in EP-A-0 215 555.
In addition to the problems associated with the use of RHM materials, the cell design employing multiple current collector bars or posts of relatively small cross-section penetrating through the cell lining has many inherent drawbacks since each current collector must carry a high current and the failure of any single current collector can lead to a total cell failure.
A number of proposals have been made for alternative cell designs having carbon cell bottoms in conjunction with inert materials underneath and/or at the sides of the carbon. See, for example, U.S. Pat. Nos. 3,390,071, 4,592,820, 4,673,481 and 4,619,750. Side-entry current feeder designs have also been proposed, e.g. in U.S. Pat. No. 3,370,071, but such designs have not found acceptance on account of a number of inherent drawbacks. There has been also a proposal in UK-A-2 076 021 to provide dividers of insulating material that subdivide the liquid aluminum cathode so that its effective surface area is somewhat less than that of facing dimensionally stable anodes, with a view to improving the anode lifetime. This arrangement, however, complicates the cell bottom and adds to its cost.
UK-A-1 206 604 has disclosed carbon blocks which protrude above a cell lining for the purpose of collecting sludge on the cell bottom. This design is, however, confined to deep pool operation and the protruding carbon elevations are subject to erosion.
The problems associated with replacing the carbon bottoms of aluminum reduction cells have thus not been resolved in a satisfactory manner, so that carbon cell bottoms continue to be the industry standard.